Auxiliary machines of automobiles, such as an air compressor and an alternator, are driven by using a belt power transmission device with an engine as a driving source. As a power transmission belt of the belt power transmission device, there has been frequently used a V-ribbed belt which contains an extension layer that forms a belt back face, a compression layer provided on one side of the extension layer, and a tension member that is embedded between the extension layer and the compression layer and extends in the belt circumferential length direction, and which has a plurality of V-shaped ribs formed in the compression layer and extending in the belt circumferential length direction, which provide a frictional power transmission surface with a pulley.
The V-ribbed belt includes one in which the formation of the ribs is performed by grinding and one in which the parts are molded in a mold. In the one in which the ribs are molded in a mold, since a grinding step is not required and a fabric to be mentioned later can be fixed to the frictional power transmission surface at the time of molding, the production process becomes simple and the yield of rubber and the like that form the belt body can be greatly improved.
In recent years, there are severe requirements for quietness in the automotive field and measures to suppress noise generation have been requested also for the V-ribbed belt for driving the auxiliary machines. The cause of the noise generation resulting from this belt includes a stick slip with a pulley, which occurs at the time of large fluctuation in the belt speed or under high load conditions. During driving in rainy weather and the like, water enters into the engine compartment and the water adheres between the belt and the pulley to lower the friction coefficient of the belt, so that slip noise may be generated.
Furthermore, for the V-ribbed belt for driving auxiliary machines, it is required to have a consistently high power transmission performance regardless of weather conditions. When rubber is exposed on the frictional power transmission surface, there is a problem that the friction coefficient is high in a dry state. Also, since the friction coefficient is considerably decreased due to a water film formed at the interface in a wet state, there is a problem that, when water enters into the engine compartment during driving in rainy weather or the like, the water adheres between the belt and the pulley to lower the friction coefficient of the belt and stick slip noise (squeak noise) is generated. Also, the rubber has a problem that deterioration with time is prone to occur.
Against these problems, there have been known a method of blending short fibers into the compression layer that forms the frictional power transmission surface and making the short fibers expose on the frictional power transmission surface, and a method of covering the frictional power transmission surface with a fabric formed of fibers. In the V-ribbed belt in which the ribs are molded with a mold, the method of covering the frictional power transmission surface with a fabric is suitable since the fabric can be fixed on the frictional power transmission surface simultaneously at the time of molding.
The fabric includes woven fabrics prepared by weaving and knitted fabrics prepared by knitting. The knitted fabrics (see, e.g., Patent Documents 1 and 2) have flexibility that enables easy fitting to the frictional power transmission surface on which concavities and convexities are formed by a plurality of the ribs, and also have an advantage that stretchability so as to stretch by following the deformation of the belt body can be easily secured.
In the one described in Patent Document 1, the knitted fabric is obtained by warp knitting with two different yarns, in which the first yarn is composed of a filament of a polyamide or the like having a higher rate than 5N/1000 dtex at the time of 5% elongation and the second yarn is composed of a filament of an elastic polyurethane or the like having a lower rate than 5 cN/1000 dtex at the time of 5% elongation. It also discloses that a flock layer in which short fibers of cotton or the like are subjected to flocking may be provided.
In the one described in Patent Document 2, the knitted fabric contains an elastic yarn of a polyurethane or the like and at least one non-elastic yarn, in which the non-elastic yarn contains a cellulose-based fiber or yarn of cotton or the like.